1. Field of the Invention
The present invention relates to a capacitive device comprising a solid electrolyte therein and utilizing an electric double layer formed between the solid electrolyte and a polarization electrode.
2. Description of the Priort Art
Ag.sup.+ ion conductive solid electrolytes such as RbAg.sub.4 I.sub.5, NR.sub.4 Ag.sub.4 I.sub.5, Ag.sub.3 SI and Ag.sub.6 I.sub.4 WO.sub.4 have been known as substances showing high ionic conductivities at room temperature. Such solid electrolytes can be prepared from AgI by substituting it partially with an anion and/or a cation, and have been employed for capacitive devices. However, such conventional capacitive devices employing Ag.sup.30 ion conductive solid electrolytes have a shortcoming that they are very expensive owing to expensive materials such as silver and silver salt. Therefore, they have been not widely employed.
In recent years, various high Cu.sup.+ ion conductive solid electrolytes have been developed. But the so far developed electrolytes with high ionic conductivities employing an organic substance have the following shortcomings, when they are employed for capacitive devices. Such capacitive devices have narrow temperature ranges in the device operations, since the employed organic substance is not sufficiently heat resistive and since an electronic conductivity becomes extremely high at a high temperature range (the increase of the electronic conductivity should be avoided, since charges once stored at the interface between a solid electrolyte and a polarization electrode are liable to disappear into the solid electrolyte by electron conduction). Further, the capacitive devices using Cu counter electrode have the following shortcoming; such capacitive devices have a short cycle life, since Cu dendrite deposited on the counter electrode causes a leak current flowing through the precipitated and deposited compound (in other words, ionic conduction does not play a primal role at this stage). Accordingly, a charge storage capability is worse than that of the conventional capacitive devices containing silver salt.
It has been proposed in a U.S. patent application Ser. No. 105,855, filed on Dec. 20, 1979, now U.S. Pat. No. 4,363,079, assigned to the assignee of the present invention) to provide a solid state electric double layer capacitor comprising a polarization electrode, a counter electrode, optionally, in addition, a reference electrode, and a solid electrolyte disposed at least between the polarization electrode and the counter electrode, the polarization electrode and the counter electrode comprising a mixture of cuprous sulfide and the solid electrolyte, the solid electrolyte comprising CuCl substituted 1/5 of Cu.sup.+ ions thereof with Rb.sup.+ ions and 1/4 to 1/3 of Cl.sup.- ions thereof with I.sup.- ions in CuCl. The recently proposed solid state electric double layer capacitor has more superior device characteristics than those of the conventional capacitive devices employing a solid electrolyte containing silver salt.
It is also proposed in the senior application mentioned above to employ a counter electrode comprising Cu and Cu.sub.2 S in a proportion of 60-80 percent by weight for Cu and 40-20 percent by weight of Cu.sub.2 S. Capacitive devices employing the counter electrode comprising Cu and Cu.sub.2 S can overcome the following conventionally known problems, to be described below. In general, when a non-polarization electrode, i.e. counter electrode is made of Cu alone, the capacitors are not usable for long hours. The reason for the inferior device characteristics is that dissolving reaction of Cu has a limit in an electrokinetic point of view. The precipitated Cu can not be dissolved reversibly, and the precipitated Cu is gradually accumulated thus resulting in electric breakdown of the capacitors. On the other hand, when reagent containing Cu.sub.2 S is employed for the non-polarization electrode, the dissolving reaction of Cu has a higher limit in the electrokinetic point of view (in other words, a limiting current becomes larger) and potential variations at the counter electrode become smaller than the case of the Cu counter electrode, but a rest potential of the Cu.sub.2 S counter electrode becomes noble by about 310 mV in comparison with the Cu potential. This causes lowering of an output voltage by the amount of the noble potential. This adverse lowering of the output voltage in the conventional solid state electrolyte capacitors can be solved by employing the counter electrode which comprises Cu and Cu.sub.2 S mixed with each other in a proportion disclosed in the senior application described above.
It has been revealed during the course of further research activities after filing the senior patent application that although the solid state electric double layer capacitors employing the proposed counter electrode have an advantageous feature in that the counter electrode potential can be set to nearly close to the Cu potential, the potential variations at the counter electrode become larger when the capacitors are operated at a low temperature.
It has been proposed in another senior application (Japanese published unexamined patent gazette Sho 52-19256) to carry out a heat treatment in the air for constituents of polarization electrodes comprising active charcoal and a solid electrolyte, for the purpose of dissolving the solid electrolyte well and putting it into porous spaces of the active charcoal. Since carbon contained in the polarization electrode is liable to be oxidized when the polarization electrode is heat-treated in the air, the heating temperature is limited to be under 180.degree. C. Solid state electric double layer capacitors employing the polarization electrode material heat-treated at a temperature under 180.degree. C. have inferior device characteristics in that a capacitance obtainable at 20.degree. C. becomes smaller by 20-30% at -25.degree. C., and in that capacitance variations with time lapse are observed to be on the same order when the capacitance is measured after 200 hours with voltage application at 70.degree. C.
On the other hand, it is necessary to employ electrochemically stable materials, i.e. materials with low ionization tendencies, for conductive materials used for the current collectors of the polarization and non-polarization electrodes, and for the purpose of electrically connecting unit cells with each other. Accordingly, the conventionally employed materials have been noble metals such as gold or platinum, and alloy metals between noble metal(s) and chromium. Evaporation or sputtering of the noble metals have been conventionally employed to form current collectors on the polarization and non-polarization electrodes. Therefore, the conventional production steps for solid state electric double layer capacitors have had problems not only in material costs, but also in production step costs.